This invention relates to loudspeakers, and more particularly to an improved bass reflex type of speaker housing or enclosure, and a method of tuning the housing to its associated speaker.
When a speaker is energized, its cone reciprocates or vibrates at a frequency which varies with the signal input to the speaker coil. When an unmounted or unbaffled speaker is operated in a so-called "free air" mode, it exhibits a sudden increase in impedance as it approaches its resonant frequency. This impedance value peaks at resonant frequency and then abruptly falls off at higher frequencies. To dampen this peak and to provide a more uniform impedance response, it is customary to mount the speaker in some form of housing, so that the air in the housing will tend to dampen this peak impedance.
In its simplest form this housing may be a closed box with the speaker mounted or suspended in an opening in one wall thereof. This construction causes the amplitude of the peak impedance to be lowered, and to occur at a different frequency, thus changing the resonant frequency of the speaker as compared to its "free air" mode of operation.
Another type of speaker housing, and the one with which this invention is primarily concerned, is known as a bass reflex or ported enclosure. Typically this enclosure includes a hole or port in one of its walls, usually the wall or speaker panel upon which the speaker cone is mounted. The enclosure itself, as represented by the air therein, thus forms a resonator, and permits some of the air from within the enclosure to be driven or forced in and out of the port during vibration of the speaker cone. Air can thus be considered to vibrate like a piston in the port, sometimes vibrating at the same frequency as the speaker cone, and at other times being out of phase with the cone frequency. Ideally, however, the frequency of this air vibration is tuned to the resonant frequency of the speaker.
As is well known by those skilled in the art, the vibrating cone of a bass reflex mounted speaker has two resonant frequencies which are lower and higher, respectively, than the normal or "free air" resonant frequency of the speaker in its unmounted or freely suspended mode. For tuning purposes, only the upper resonant frequency need be tuned to the associated enclosure, because the lower resonant frequency is too low to be of any concern. (For reference, see How To Design, Build & Test Complete Speaker Systems by David B. Weems, 1st Ed., Tab Books of Blue Ridge Summit, Pa.)
Heretofore it has been customary to tune a speaker and its associated bass reflex enclosure either by varying the size of the port, or the overall size of the enclosure itself. One difficulty with these methods of tuning is that extreme care must be taken to employ the right size of housing and the correct port diameter, because any slight adjustment of one parameter usually requires a corresponding adjustment of the other.